The first detector in a television signal receiver converts radio-frequency (RF) signals in a selected one of the television broadcast channels, which channels occupy various 6-MHz-wide portions of the electromagnetic wave frequency spectrum, to an intermediate-frequency (IF) signals in one particular 6-MHz-wide portion of that spectrum. This conversion is typically carried out by superheterodyning the RF signals, which is to say mixing the RF signals with local oscillations from a first local oscillator oscillating at a frequency substantially higher than the frequencies in the television channel of highest frequency, which mixing is done by linear multiplication in a first mixer. The first mixer is preferably of doubly balanced type. The first detector is used to convert a selected RF signal to IF signal in order that up to 60 dB or more amplification can be done in that particular 6-MHz-wide portion of that spectrum using IF amplifiers with fixed, rather than variable, tuning. Amplification of the received signals is necessary to raise them to power levels required for further signal detection operations, such as video detection and sound detection in the case of analog TV signals, and such as symbol decoding in the case of digital TV signals. The first detector usually includes variable tuning elements in the form of pre-selection filter circuitry for the RF signals to select among the various 6-MHz-wide television channels and in the further form of elements for determining the frequency of the local oscillations used for super-heterodyning the RF signals.
In analog TV signal receivers, which generally employ single-conversion radio receivers, the frequency of the oscillations supplied by the first local oscillator is often fine-tuned in response to an electric fine-tuning signal. This fine-tuning signal is generated by an automatic fine-tuning (AFT) feedback loop, which includes a bandpass filter responsive to the video carrier component of an IF amplifier response and an AFT detector that generates the electric fine-tuning signal. The AFT detector typically includes a limiter amplifier and a frequency discriminator tuned for prescribed video carrier frequency as translated to IF by the first detector. AFT is done to adjust the IF signal, so that the frequency-modulated (FM) audio carrier component of the selected TV channel as it appears in the amplified IF signal supplied to the video detector falls into the in-channel sound trap filter, and so that the FM audio carrier of an adjacent TV channel next below in frequency as it appears in the amplified IF signal supplied to the video detector falls into the adjacent-channel sound trap filter. Also, the IF amplifier amplitude response is rolled off 6 dB at video carrier frequency to provide matched filtering for the vestigial sideband (VSB) filter in the analog TV transmitter, so the IF signal should be aligned such that this amplitude equalization is correctly performed.
In some analog TV receivers of recent design first local oscillator signals are generated using a frequency synthesizer in which the first local oscillator signals are generated with frequency regulated in adjustable offset from the fixed frequency of a standard oscillator. This is advantageous if the frequency of the first local oscillations is to be fine-tuned in response to an electric fine-tuning signal, since electric fine-tuning of the frequency of the standard oscillator can be done (e.g., by using a varactor diode in an LC tank circuit) with the sensitivity of absolute frequency adjustment of first local oscillations to electric fine-tuning signal being constant no matter what the nominal frequency of the first local oscillator signals is.
Digital television (DTV) receivers known in the prior art did not use AFT of the first local oscillator. The data carrier of the VSB signal is nominally located 310 kHz from the lower limit frequency of the 6 MHz TV broadcast channel, and the uppermost sideband nominally extends to 310 kHz from the lower limit frequency of the 6 MHz TV broadcast channel. The IF amplifier bandwidths have been 6 MHz wide with less than a 1 dB ripple in amplitude response across the passband, so critical (fine) tuning has not been required.
Automatic fine tuning is desirable in a receiver for digital television (DTV) signals that uses a frequency synthesizer for tuning, even if the local oscillators in the receiver have extremely good frequency stability and are accurately tuned to prescribed nominal values. This is because transmitter carrier frequencies can be purposely tuned to depart as much as eight kilohertz from a frequency 310 kilohertz above a multiple of six megahertz. This is done to offset the pilot carrier frequency of a DTV signal from the color subcarrier of an NTSC signal on the lower adjacent channel by an odd multiple of one-half NTSC horizontal scan frequency, for reducing the visibility of color beat responses to the DTV pilot carrier.
Allowed patent application Ser. No. 08/822,736 points out that the introduction of in-channel and adjacent-channel sound trap filtering into the IF amplifier chain of a DTV receiver is advantageous in reducing the artifacts of NTSC signal interference that will accompany symbol codes recovered in a DTV receiver, during a transition era in which analog TV broadcasting continues still to be done. If such sound trap filtering is introduced into the IF amplifier chain amplifying DTV signals, it becomes advantageous to use AFT of the first local oscillator in a DTV receiver during the reception of DTV signals, patent application serial No. 08/822,736 points out.
Television signal receivers capable of receiving both digital television (DTV) signals transmitted in accordance with the ATSC broadcast standard and analog TV signals transmitted in accordance with the NTSC broadcast standard are described in the inventor""s U.S. patent applications Ser. Nos. 08/825,711 and 08/820,193 filed Mar. 19, 1997, and respectively entitled RADIO RECEIVER DETECTING DIGITAL AND ANALOG TELEVISION RADIO-FREQUENCY SIGNALS WITH SINGLE FIRST DETECTOR and DIGITAL-AND-ANALOG-TV-SIGNAL RECEIVERS, EACH WITH SINGLE FIRST DETECTOR AND SHARED HIGH-BAND I-F AMPLIFICATION.
These applications are incorporated herein by reference for providing details of receiver construction not directly related to developing and utilizing automatic fine-tuning signals. The TV receivers described in these applications are of plural-conversion type, each using a single first detector both DTV signals and analog TV signals. The first detector generates ultra-high-frequency (UHF) intermediate-frequency signals. There are differing requirements for IF amplification of DTV signals and IF amplification of analog TV signals, so each of these TV receivers uses different IF amplifier chains for analog TV signals and for DTV signals. In some of these TV receivers the different IF amplifier chains for analog TV signals and for DTV signals do share some IF amplifier stages, however. The first detector generates ultra-high-frequency (UHF) intermediate-frequency signals which are subjected to frequency-selective filtering with bandwidth(s) just sufficient to pass the broadcast signal that is currently selected for reception, so automatic fine tuning (AFT) of first local oscillator signal is a practical necessity. When a single first detector is used both for DTV signal reception and for analog TV signal reception, problems of how properly to develop AFT signals arise. Solutions to these problems are described in patent application Ser. No. 08/822,736.
The derivation of AFT signals from the response of the IF amplifier chain for the amplitude-modulated NTSC video carrier is known from experience in analog TV signal receiver design to have problems, which problems arise owing to the 6 dB roll-off of video carrier that is customary for match filtering the vestigial sideband filter at the transmitter and at the same time helping in the design of the adjacent channel sound trapping. Accordingly, in an analog TV signal receiver design using a quasi-parallel IF amplifier chain for intercarrier sound, the bandpass filter for selecting video carrier to the AFT detector is connected to receive the response of the quasi-parallel IF amplifier chain, rather than the response of the IF amplifier chain for the amplitude-modulated NTSC video carrier supplied to the video detector.
The vestigial-sideband signal used for digital television broadcasting differs from the vestigial-sideband signal used in analog television broadcasting in that the spectrum roll-off of the upper frequencies of the amplitude modulation begins at a frequency below carrier frequency, rather than above carrier frequency. So, a substantially flat amplitude response through carrier region is not available, even with a quasi-parallel IF amplifier chain. Accordingly, it is here pointed out, the bandpass filter used for extracting the frequencies near DTV carrier frequency for application to the AFT detector should have a tilted amplitude response in passband that compensates for the roll-off of the DTV signal through the carrier region. This is done to secure sideband symmetry in the region around carrier, so that low-frequency modulation components will not affect the AFT. This is necessary because the ATSC standard does not provide for these low-frequency modulation components near carrier being suppressed together with the carrier. Allowing the low-frequency modulation components to affect AFT causes carrier phase modulation that is deleterious to decoding baseband symbol code properly.
Patent application Ser. No. 08/822,736 directed particular attention to the problems of developing fine-tuning signals to the first detector in the radio receiver portions of TV signal receivers for receiving DTV signals during the transition era in which analog TV broadcasting continues still to be done. However, automatic fine-tuning (AFT) of the local oscillator used in the first detector will still continue to be of importance in certain types of TV signal receivers for receiving DTV signals after the transition eraxe2x80x94that is, when analog TV broadcasting is no longer done.
The ATSC data broadcast standard for terrestrial through-the-air television broadcasting prescribes a system channel response that is a linear-phase raised cosine filter response that is 6 dB down at 5.38 MHz bandwidth. This establishes a Nyquist slope at the higher-frequency end of the system channel response, and this establishes a corresponding slope 6 dB down at carrier frequency at the lower-frequency end of the system channel response. While not explicit in the published standard, half the higher-frequency roll-off is to be done at the transmitter, and half the lower-frequency roll-off is to be done at the transmitter. The remaining portions of these prescribed frequency roll-offs are to be accommodated at the receiver.
The inventor discerns that in most receivers for DTV signals, the remaining portions of these prescribed frequency roll-offs will be provided by filtering of the intermediate-frequency signals. The system channel response apparently was specified by persons more familiar with QAM data transmission than with VSB data transmission, since the frequency roll-off at the side of the channel proximate to the carrier frequency is referred to as a Nyquist slope in ATSC publications, as well as the frequency roll-off at the side of the channel distal from the carrier frequency. In QAM, which has both upper and lower sidebands that extend over respective frequency ranges each equal to half symbol rate, both of the frequency roll-offs are Nyquist slopes that affect the ability to distinguish changes between consecutive symbols. In a QAM data transmission system in which the Nyquist slope is only partially provided for at the transmitter, if the receiver uses the Viterbi algorithm for symbol decoding, it is known that the receiver need not complete the filtering for achieving the Nyquist slopes in order to decode the data symbols successfully.
In VSB data transmission the frequency roll-off at the side of the channel proximate to the carrier frequency is not a Nyquist slope, since it does not affect the ability to distinguish changes between consecutive symbols. In VSB data transmission, the frequency roll-off at the side of the channel proximate to the carrier frequency affects the regeneration of carrier in the receiver and affects the demodulated baseband response at frequencies close to zero frequency. The use of the Viterbi algorithm for symbol decoding does not help appreciably in solving these VSB reception problems. Error in the demodulated baseband response at frequencies close to zero frequency can be compensated for in the channel equalization circuitry and consequently is usually not of much concern. The greater concern is avoiding response to the pilot carrier being rolled off too much. Too much reduction in the pilot carrier amplitude adversely affects the automatic frequency-and-phase-lock loop of the local oscillator used for carrier regeneration, causing carrier jitter and increasing the likelihood of loss of synchronization of receiver carrier with transmitter carrier.
DTV receivers which do not introduce further roll-off of the system channel response in the RF and IF amplifiers avoid the problem of response to the pilot carrier being rolled off too much. However, such DTV receivers are undesirable in that the noise in the frequency spectrum extending up from carrier frequency is demodulated, reducing signal-to-noise in the baseband symbol coding recovered in the synchrodyne to baseband. Furthermore, in situations in which DTV signals are transmitted in adjacent channels, especially in those particular situations in which the adjacent-channel signal is strong relative to the signal selected for reception, it is desirable not to sacrifice the selectivity against adjacent-channel signals provided by rolling off to the specified system channel response. The adjacent-channel signals should be reduced enough in amplitude, if possible, that they do not cause intermodulation distortion with the signal selected for reception. Baseband signal resulting from demodulation of these adjacent-channel signals is above half-symbol frequency and can be rejected by lowpass filtering of the baseband symbol code. But intermodulation distortion occupies in part the same spectral region as baseband symbol code and cannot be separated therefrom by frequency-selective filtering. In DTV receivers that roll off IF amplifier response to the specified system channel response, to help reject adjacent-channel signals, AFT is desirable to avoid either response to the pilot carrier or response to half symbol frequency being rolled off too much.
DTV receivers are contemplated in which pilot frequency is boosted respective to other components in the amplified DTV IF signal supplied for demodulation in synchrodyning to baseband procedures. This boosting of pilot frequency reduces the carrier jitter and decreases the likelihood of loss of synchronization of receiver carrier with transmitter carrier. The boosting of pilot carrier amplitude has to be done with narrow selectivity and can be implemented by surface-acoustic-wave (SAW) filters in the IF amplifier chain for DTV signal. Automatic fine tuning is desirable in such a receiver, to maintain the pilot carrier of the received DTV signal as translated to intermediate frequency at the frequency which is boosted in the IF amplifier response supplied for demodulation.
In a superheterodyne radio receiver designed for DTV reception, the IF amplifier chain for the VSB data modulation includes filtering having selective response to different portions of a received DTV signal, as translated to intermediate frequencies by a first detector; and there is automatic fine tuning of a local oscillator included in the first detector to assure that the received DTV signal, as translated to intermediate frequencies by the first detector, is in proper alignment with the filtering having selective response to different portions of the received DTV signal. The bandpass filter used for extracting the frequencies near DTV carrier frequency for application to the AFT detector has a tilted amplitude response in passband that compensates for the roll-off of the DTV signal through the carrier region.